This invention relates to novel electroactive energy storing polyacetylene-co-polysulfur (PAS) materials of general formula (C.sub.2 S.sub.x).sub.n wherein x is greater than 1 to about 100, and n is equal to or greater than 2. This invention also relates to novel rechargeable electrochemical cells containing positive electrode materials comprised of said polyacetylene-co-polysulfur materials with improved storage capacity at ambient and sub-ambient temperatures.
Batteries are used in almost all portable consumer electronic products from flash lights to lap top computers. Over the years, considerable interest has been shown in developing lighter weight high energy-density rechargeable batteries for many applications including electric vehicles. In this regard, thin film solid state batteries using the polyacetylene-co-polysulfur cathode materials of this invention are particularly well suited for use in many consumer applications because of their high energy to weight ratio.
Two main types of cathode materials used in the manufacture of thin film lithium and sodium batteries are known in the art. The first materials include transition metal chalcogenides, such as titanium disulfide with alkali metals as the anode. For example, among the cathode active chalcogenides, U.S. Pat. No. 4,049,879 lists transition metal phosphorous chalcogenides. Other U.S. patents, such as U.S. Pat. Nos. 4,143,214, 4,152,491 and 4,664,991 describe cells wherein the cathode is a carbon/sulfur based material, generally of the C.sub.x S formula where x is typically 10 or larger.
U.S. Pat. No. 4,143,294 to Chang, et al. describes cells having cathodes containing C.sub.x S wherein x is a numerical value from about 4 to about 50. U.S. Pat. No. 4,152,491 to Chang et al. relates to electric current producing cells where the cathode-active materials include one or more polymer compounds having a plurality of carbon monosulfide units. The carbon monosulfide unit is generally described as (CS).sub.x, wherein x is an integer of at least 5, and may be at least 50, and is preferably at least 100. In both cells developed by Chang, et al. the energy storage capacity is limited because there is a low density of sulfur-sulfur bonds.
U.S. Pat. No. 4,664,991 to Perichaud, et al. describes a substance containing a one-dimensional electric conducting polymer and at least one polysulfurated chain forming a charge-transfer complex with the polymer. Perichaud, et al. use a material which has two components. One is the conducting polymer, which is selected from a group consisting of polyacetylenes, polyparaphenylenes, polythiophenes, polypyrroles, polyanilines and their substituted derivatives. The other is a polysulfurated chain which is in a charge transfer relation to the conducting polymer. The polysulfurated chain is formed by high temperature heating of sulfur with the conjugated polymer. As a result of using this material, the cell of Perichaud, et al. exhibits a fairly low voltage of only 2.0 V against lithium.
In a related approach, a PCT application (PCT/FR84/00202) of Armand et al. describes derivatives of polyacetylene-co-polysulfurs comprising units of R.sub.x (CS.sub.m).sub.n wherein R is hydrogen, alkali metal, or transition metal, x has values ranging from 0 to values equal to the valence of the metal ion used, values for m range from greater than 0 to less than or equal to 1, and n is unspecified. Structures proposed for these materials are of the type: ##STR1##
wherein such materials are derived from the reduction of polytetrafluoroethylene or polytrifluorochloroethylene with alkali metals in the presence of sulfur, or by the sulfuration of polyacetylene with vapors of sulfur monochloride at 220.degree. C. Although these materials are electrochemically active, they suffer from low storage capacity owing to low S/C ratios and a limited number of S--S bonds in the materials. These materials can have a considerable amount of residual hydrogen, fluorine, and chlorine atoms in their backbones depending on the method of synthesis.
It is reported in a series of papers by B. A. Dogadkin and A. A. Dontsov [Vysokomol. Soedin, 3(11), 1746 (1961); Vysokomol. Soedin, 7(11), 1841 (1965); and Dokl. Akad. Nauk SSSR, 138(6), 1349 (1961)] that the interaction of polyethylene with sulfur in sealed reaction vessels at 200-240.degree. C. is accompanied by the incorporation of sulfur into C--H bonds with subsequent crosslinking between the polyethylene chains. The maximum amount of sulfur incorporated does not depend on temperature and is only about 3.7% by weight. The resulting crosslinked polymer is comprised of a substantially saturated polymer (polyethylene) backbone. There is no mention of any electrochemical activity for these materials.
U.S. Pat. Nos. 4,833,048 and 4,917,974 to De Jonghe, et al. describe a class of cathode materials made of organo-sulfur compounds of the formula (R(S).sub.y).sub.n where y=1 to 6; n=2 to 20, and R is one or more different aliphatic or aromatic organic moieties having one to twenty carbon atoms. One or more oxygen, sulfur, nitrogen or fluorine atoms associated with the chain can also be included when R is an aliphatic chain. The aliphatic chain may be linear or branched, saturated or unsaturated. The aliphatic chain or the aromatic rings may have substituent groups. The preferred form of the cathode material is a simple dimer or (RS).sub.2. When the organic moiety R is a straight or a branched aliphatic chain, such moieties as alkyl, alkenyl, alkynyl, alkoxyalkyl, alkythioalkyl, or aminoalkyl groups and their fluorine derivatives may be included. When the organic moiety comprises an aromatic group, the group may comprise an aryl, arylalkyl or alkylaryl group, including fluorine substituted derivatives, and the ring may also contain one or more nitrogen, sulfur, or oxygen heteroatoms as well.
In the cell developed by De Jonghe, et al. the main cathode reaction during discharge of the battery is the breaking and reforming of disulfide bonds. The breaking of a disulfide bond is associated with the formation of an RS.sup.- M.sup.+ ionic complex. The organo-sulfur materials investigated by De Jonghe, et al. undergo polymerization (dimerization) and de-polymerization (disulfide cleavage) upon the formation and breaking of the disulfide bonds. The de-polymerization which occurs during the discharging of the cell results in lower weight monomeric species which can dissolve into the electrolyte, thereby severely reducing the utility of the organo-sulfur material as cathode-active material. The result is an unsatisfactory cycle life having a maximum of about 200 deep discharge-charge cycles, more typically less than 100 cycles as described in J. Electrochem. Soc., Vol. 138, pp. 1891-1895 (1991). In particular, the organo-sulfur materials developed by De Jonghe, et al., are highly unstable in the presence of high conductivity liquid, plasticized polymer, or gel electrolytes.
A significant additional drawback with the organo-sulfur materials developed by De Jonghe, et al. is the slow kinetics of oxidation and reduction at ambient temperatures, severely reducing the power output of cells incorporating cathodes made with these organo-sulfur materials. The slow kinetics result from the oxidation and reduction being related to the formation and breaking, respectively, of disulfide bonds on non-conjugated, non-conductive materials.
Despite the various approaches proposed for organo-sulfur cathode materials, there remains a need for inexpensive cathode materials having high storage capacity, high discharge rates and very long cycle lives at ambient and sub-ambient temperatures.
It is, therefore, a primary object of this invention to provide new polyacetylene-co-polysulfur based cathode materials for thin film solid state batteries which are inexpensive, yet avoid the limitations existing in the prior art, while offering performance characteristics much higher than those of known materials.
It is another object of this invention to provide new cathode materials having as the active material polyacetylene-co-polysulfur (PAS) polymers which do not undergo polymerization and de-polymerization upon oxidation and reduction.
It is yet another object of this invention to provide a method of making a solid state rechargeable battery including the novel cathode of the invention.